1. Field
Example embodiments relate to a radio transmitter-receiver system, for example, to a method and apparatus for managing clock signals in an ultra-wide band (UWB) reception system.
2. Description of the Related Art
A UWB reception system is a type radio technology that enables data transmission in ultrahigh frequency with relatively low power. The UWM system may effectively utilize frequency resources by sharing frequency spectrums of traditional communication systems, thereby providing increased reliability. The characteristics of low power and increased reliability may make UWB technology ideal for radio frequency sensitive environments, such as, collision protecting equipment for flying objects, an altimeter measuring an altitude from the ground at an aviation facility and an airplane, and position chase, for example. Moreover, UWB technology is used in the field of medical science, for monitoring patients' conditions, and examining the physical condition of embryos, for example. Because a conventional UWB system generally uses an ultrahigh bandwidth, interference with radio frequencies used in a global positioning system (GPS) and mobile communication networks may cause problems. Therefore, the U.S. Federal Communications Commission (FCC) tightly controls the commercial utilization of UWB, but recently has allowed UWB technology to be commercial available under certain conditions.
The FCC defines the UWB system as a communication mode occupying a frequency bandwidth over 500 MHz or 20% of a center frequency. Presently, the FCC provides a limitation regarding transmission signal power to the frequency band of 3.1˜10.6 GHz for communication. Because a UWB signal operates in a wide frequency band, it may have a relatively small value of power spectral density in a frequency domain. Due to the low power spectral density, the UWM system under FCC regulation may have relatively minor interference problems with other communication signals. Previous conventional UWB technologies utilized short pulses within a broadband, but now Multi-Band Orthogonal Frequency-Division Multiplexing (MB-OFDM) and direct sequence UWB (DS-UWB) modes may be considered ideal for the IEEE P802.15.3a UWB communication standard. The MB-OFDM mode may utilize Time-Frequency (TF) hopping patterns for offering a multiple Simultaneous Operating Piconet (multi-SOP) by satisfying the transmission signal power provision proposed by the FCC to minimize power consumption.
The conventional MB-OFDM mode differs from the traditional OFDM mode because the frequency in a MB-FDM mode may be modified every OFDM symbol in accordance with TF hopping pattern. The conventional MB-OFDM mode and the traditional OFDM mode are similar because the MB-OFDM or OFDM mode may operate to transmit data in parallel by sub-carriers. Therefore, the UWB system with the MB-OFDM mode may be sensitive to a sub-carrier offset similar to a single carrier transmission system. In other words, if there is a sub-carrier frequency offset between a transmitter and a receiver, it may degrade the orthogonality between sub-carrier waves, thereby inducing Inter-Carrier Interference (ICI).
Within the UWB radio communication system, the standardization of the IEEE P802.15.3a provision may be progressing towards ultrahigh-speed radio data transmission of 480 Mbps in a close distance of 3 meters.
FIG. 1 is a block diagram illustrating a conventional UWB receiver 100 operating in MB-OFDM mode. Referring to FIG. 1, the UWB receiver 100 may receive a signal r(t) through an antenna. An amplifier (LNA) 110 may amplify the received signal r(t). The amplified received signal may be divided into inphase I and quadrature Q components by using cosine and sine waves at the carrier frequency domain. The inphase I component may be converted into a digital signal through a low pass filter (LPF) 120, a voltage gain amplifier (VGA) 130, and an analog-to-digital converter (ADC) 140. Similarly, the quadrature Q may be converted into a digital signal through a low pass filter (LPF) 121, a voltage gain amplifier (VGA) 131, and an analog-to-digital converter (ADC) 141. The inphase I and quadrature Q components may be applied to a fast Fourier transformer (FFT) 150 and a frequency equalizer (FEQ) 160 to decode the digital signals in OFDM code or symbols. The FFT 150 and the FEQ 160 maybe operable in OFDM baseband. Furthermore, the frequency and time equalizer may be activated in the OFDM baseband block.
The OFDM coded symbol signal from the FFT 150 and FEQ 160 may be applied to a bit-level processing unit 170. After the OFDM coded symbol signal is processed, the output from the bit-level processing unit 170 may be a bit level data chain. Processing the OFDM coded signals in the bit-level processing unit 170 may include de-interleaving, de-puncturing, and Viterbi decoding operations. The UWB system may include a 6-OFDM mode, where processing the OFDM coded signals may be executed in the unit of 6 symbols (interleaving depth). After the OFDM coded signals are processed, the bit-level processing unit 170 generates an output data chain or Output Data.
FIG. 2 illustrates a frame structure of 6-OFDM data of a conventional UWB data packet. Referring to FIG. 2, the frame of a 6-OFDM data packet 200 may be divided into a header and a physical layer service data unit (PSDU) 250. The header may include general information for receiving the PSDU 250. The PSDU 250 may include information to be transmitted. The header may include a preamble 210, a start frame delimiter (SFD) 220, a frame length sign 230, and a surplus field 240. The preamble 210 may indicate the synchronization. The SFD 220 may indicate an end of the preamble 210. The frame length sign 230 may indicate a length of the PSDU 250. In a conventional 6-OFDM system, the header may include approximately 256 bytes and the PSDU 250 may include approximately 0 to 4095 bytes. In the 6-OFDM receiver, a demodulation method and a code rate may be modified in accordance with a transmission speed or data rate of the PSDU 250.
FIG. 3 illustrates parameters of a conventional receiver according to data rates of the PSDU. Referring to FIG. 3, when the data rate of the PSDU 250 is over 320 Mbps, the 6-OFDM system may utilize Dual-Carrier Modulation (DCM) as a modulation mode. When the data rate of the PSDU 250 is under 320 Mbps, the 6-OFDM system may utilize Quadrature Phase Shift Keying (QPSK) as a modulation mode. By utilizing duel modulation modes, the 6-OFDM system may operate with frequency diversity in the environment of selective frequency fading.
Referring to FIG. 3, the 6-OFDM system may operate according to eight data rates or data transmission rate modes. Because the receiver must support all data process operations (e.g., de-interleaving, de-puncturing, Viterbi decoding), the receiver may operate at a relatively high rate of speed, regardless of the eight data transmission rate modes. Consequently, the receiver may require a high frequency clock for supporting such a high data transmission rate. For example, the frequency of a drive clock of the bit-level processing unit 170 may be fixed to a designed value regardless of a data transmission rate. The designated value may be supplied to permit the maximum data transmission speed. Therefore, all function blocks of the receiver may be driven by a clock frequency of 132 MHz to support the maximum data rate of 480 Mbps. Because the receiver may require a high clock frequency for supporting the fast data transmission rate, the receiver may dissipate a relatively high amount of power. The bit-level processing unit 170 subsequent to the OFDM demodulation stage may dissipate power around 40% of the whole power consumption in the receiver.
During an idle time of the bit level processing unit, static power consumption may remain due to leakage currents of clock paths. For example, in a relatively low data transmission rate of the PSDU, the bit-level processing unit 170 may conduct a faster operation relative to the other function blocks because the clock frequency is operating may be 132 MHz. Therefore, an idle time of operation may exist in the bit-level processing unit 170, whereby static power consumption may still remain. The static and dynamic power consumption may act as a large factor of power dissipation in the UWB system.